The present invention especially relates to an image reading apparatus for imaging and reading an X-ray radiation distribution. In a conventional radiography apparatus, radiation is emitted by a radiation source toward an object as a medium to be inspected, and is intensity-modulated and scattered by the interaction between the radiation and object in correspondence with the internal structure of the object, thus forming a radiation image on a solid-state imaging element. At this time, in order to remove scattered radiation and to improve the contrast of the radiation image, a grid is placed in front of the solid-state imaging element to photograph an image.
On the other hand, an apparatus which takes correction photographs under various photographing conditions in advance, and corrects each photographed image using an image correction-photographed under a condition closest to the actual photographing condition is disclosed in Japanese Laid-Open Patent No. 2-237277.
However, the conventional apparatus does not consider the directivity of the grid placed between the solid-state imaging element and object. When an image is photographed via the grid by a method using a two-dimensional low-pass filter, if the grid direction is not taken into consideration, a problem is posed when the relative positional relationship between the grid and solid-state imaging element deviates. More specifically, when the grid is inserted, if gain correction coefficient data as a variation distribution of conversion efficiency is calculated using the two-dimensional low-pass filter, the calculated data contains unwanted data output from the grid.
FIG. 8 shows a graph of a sensor output Sij and low-pass filter output Lij in a section A for a correction data acquisition image obtained by photographing in the absence of any object so as to obtain gain correction coefficient data. If the quotient obtained by dividing the sensor output Sij by the low-pass filter output Lij is used as gain correction coefficient data (Cij) that indicates the conversion efficiency of a solid-state imaging element 1, the gain correction coefficient Cij contains the influence of a grid 2.
More specifically, since the low-pass filter output Lij underestimates the X-ray dose contributed by the low-pass filter, a point P corresponding to a plus peak of the sensor output Sij exhibits higher apparent conversion efficiency than that of the solid-state imaging element 1 at the actual point P, and a point Q corresponding to a minus peak of the sensor output Sij exhibits lower apparent conversion efficiency than that of the solid-state imaging element 1 at the actual point Q. In this way, when the relative positional relationship between the grid 2 and sensor deviates upon photographing an actual object, the outputs can no longer correspond to the gain correction coefficient Cij. On the other hand, a calculation is made under the assumption that the relative positional relationship between the grid 2 and sensor has one-to-one correspondence with gain correction coefficient data. So, an image corrected using such gain correction coefficient data cannot be corrected accurately.
When gain correction is done in units of pixels of the solid-state imaging element 1, gain correction coefficient data for such correction is required upon photographing. However, when the relative positional relationship between the solid-state imaging element 1 and grid 2 upon collecting data in image acquisition deviates from that upon actually photographing an object, accurate correction cannot be attained.
Normally, as the X-ray technician takes it for granted that the solid-state imaging element 1 and grid 2 are always parallel to each other, the relative positional relationship between the solid-state imaging element 1 and grid 2 must always be automatically adjusted in a desired direction before or upon obtaining gain correction coefficient data. At this time, in order to assure more versatile use of the apparatus, accurate gain correction coefficient data must be obtained after not only the solid-state imaging element 1 and the grid 2 are adjusted in the pixel line-up direction, but also they are finely adjusted in respect to a direction perpendicular to the pixel line-up direction and other directions.
In Japanese Laid-Open Patent No. 5-237277 above, since photographing for correction must be done under a plurality of different photographing conditions, a storage memory requires a very large capacity.